Classical MIS structure field-effect transistors, commonly called MISFETs or MOSFETs, are generally formed on a silicon substrate strongly doped to make the transistors conduct. A metal layer is deposited on one surface of the substrate so that the grid voltage can be applied. An insulating silica layer is grown on the other surface of the substrate. A semiconducting layer and the two metal contacts constituting the source and drain are applied to this silica layer. The source and drain may be in contact with the insulating layer or disposed above the semiconducting layer. It has been shown over the last decade that the insulator in the MISFET structure can be replaced by insulating polymers with dielectric constants exceeding silica, and that the semiconductor components in MISFETs can be replaced by a conjugated semiconducting polymer or organic molecules with aromatic structures of finite molecular weight. This choice of transistor components has enabled applications where requirements of flexibility over a large area are required. A particular need for such organic-FET transistors is in display applications, for example, as an option to amorphous silicon based thin film vast transistors for driving the light emitting diodes (LEDs). Another application is in development of active-matrix drive circuits printed on plastic media. The transistors in these circuits are made of plastic materials and are fabricated with a low-cost printing process that uses high-resolution rubber stamps. Their switching properties are similar to typical thin film transistors manufactured from silicon and through conventional fabrication methods, but they are mechanically flexible, rugged and lightweight.
Advances in crystal growth techniques of organic molecules with much lower degrees of defects have led to tremendous improvements of material characteristics resulting in enhanced transistor properties. The improvements in polymer-transistor characteristics over the last decade were possible due to improvement of polymers used, in terms of purity and orientation methods. Regioregular polyalkylthiophenes P3ATs have been shown to self-assemble into such films, with the molecules adopting a preferred orientation with respect to the substrate. Consequently, it has been shown that semiconducting polymers with mobilities as high as 0.1 cm2/V-s can be realized.
The photosensitivity of these polymers significantly increases upon dispersing electron acceptors such as derivatised buckminsterfullerene C60, viologen, nanoparticles of TiO2, CdS and other such materials with suitable energy levels for accepting the photogenerated electron in the polymer matrix. Device fabrication with composites of conjugated polymers and C60 as the active layer with efficient photo-induced charge transfer preventing the initial e-h recombination has resulted in efficient organic photodiodes and photovoltaic cells. A prerequisite for such an enhancement are materials with high electron affinity with a distribution in the polymer matrix such that the interparticle distances is on the order of the exciton diffusion length. In addition, the charge separation process must be fast enough to compete with the radiative and nonradiative decay pathways of the excited species, which is in the range of 100 ps to 1 ns.
One of the applications of silicon technology is in the field of image sensors. As is well known, an image sensor is a semiconductor device for sensing a light reflected from an object to generate image data. A photo-transistor, unlike a photo-diode, is a high output impedance (light-controlled) current source. Also, the output impedance of a photo-transistor can be rendered independent of the size of its photosensing junction, while the output impedance of a photo-diode cannot. Photo-transistors are more effective sensors than photo-diodes in certain applications because of these two properties. Such applications include those in which the voltage output of the loaded sensor is limited by the impedance of the sensor rather than the dynamic range of the load.
An optically activated FET using organic semiconductors without an explicit insulating layer but where the aluminum shottky gate region forms the active region has been reported. High sensitivity to light in n-channel silicon-on-insulator (SOI) metal oxide FET which operates in the inversion mode has been demonstrated. The photodetector comprises a short-channel SOI film with a negatively biased gate electrode. Under illumination, electrons generated in the semiconductor flow away from the channel region and into the drain whereas the photogenerated holes remain in the channel. Positive charges of the remaining holes bias the source-channel junction forward, leading to large drain current.